1. Field of the Invention
The present invention relates to a selective call receiving apparatus suitable for use in broadcasting or communication, and specifically in FM multiplexed broadcasting.
2. Description of the Related Art
Recently services which multiplex information, including characters and other display information, on an FM broadcast program and broadcast the resultant information have been put into practice. FIG. 10 illustrates the principle of such multiplexing of FM multiplexed broadcasting.
A sum signal ((L+R) signal; main channel signal) of a left (L) signal and right (R) signal and a difference signal ((L-R) signal) are generated from L and R audio signals. Those signals are all limited to a frequency band of 15 KHz. A carrier which belongs to the difference signal ((L-R) signal) and has a frequency of 38 KHz (2 fp), twice the frequency of a pilot signal fp, is subjected to carrier suppression amplitude modulation to generate a subchannel signal. Then, the main channel signal, the pilot signal fp having a frequency of 19 KHz, and the subchannel signal are added together to prepare a stereo composite signal for FM audio broadcasting.
A multiplexed signal is in the frequency band where the baseband frequency is equal to or greater than 53 KHz and equal to or less than 100 KHz, and has a subcarrier frequency of 76 KHz (4 fp), four times the frequency of the pilot signal. This multiplexed signal is subjected to L-MSK (Level controlled Minimum Shift Keying) modulation, and the resultant signal is frequency-multiplexed on the stereo composite signal. Therefore, a carrier of a predetermined frequency is frequency-modulated by a signal obtained by adding the main channel signal, pilot signal, subchannel signal and subcarrier frequency signal (multiplexed signal) together, thereby yielding an FM multiplexed broadcast wave.
When the modulation level of the difference signal (L-R) signal) is large, the interference on multiplexed signals increases. The interference noise to audio signals is likely to be detected when the modulation of audio signals is small. In this respect, each multiplexed signal is subjected to L-MSK modulation so that the modulation level of the multiplexed signal is changed between 4% and 10% in accordance with the modulation level of the L-R signal, as shown in FIG. 11.
The details of this technique are described in Examined Japanese Patent Publication (Kokoku) No. Hei 8-2044.
The transfer rate of multiplexed signals is 16 Kbps of which 6.83 Kbps are used as a data area and the remaining 9.17 Kbps are used for error correction.
Used in this error correction is a product code ((272, 190) shortened difference set cyclic code) obtained by breaking down a single code to symbols which are arranged in the horizontal and vertical directions, as shown in FIG. 12. The details of this technique are described in U.S. Pat. No. 4,630,271, U.S. Pat. No. 4,819,231 and U.S. Pat. No. 5,432,800.
For example, a single broadcasting station can broadcast a maximum of 256 programs as such multiplexed information.
Recently, there are paging services (selective call services) which use this FM multiplexed broadcast system.
One such above-described FM multiplexed broadcast system is called DARC (DAta Radio Channel). This system has been developed in Japan.
Other FM multiplexed broadcast systems include the RECEPTER system (HSDS=High speed Subcarrier Data System) which is locally employed in the U.S.A. and RDS (Radio Data System) which is locally employed in Europe.
In providing paging services using FM multiplexed broadcast, a receiving apparatus should always be powered on. It is therefore important to reduce the power consumption.